Machines utilizing one or more rotary shafts include bearings which must not only prevent wear at the rotating-stationary interface, but which also must prevent substantial radial and/or axial movement of the shaft. In the case of a rotary screw compressor, thrust bearings are used which prevent substantial axial movement of male and female rotors which are rotating at high speeds. In this application, it is critical to limit axial movement to keep the service life of the bearings as long as possible.
Soderlund U.S. Pat. No. 5,411,388 discloses a rotary screw machine with thrust balanced bearings. A shaft journal is supported axially by two main angular contact ball bearings arranged face-to-face with the inner races clamped toward the lower pressure end of the machine. The shaft journal is further mounted in a balancing thrust bearing spaced from the main thrust bearings. A thrust balancing device is provided between the main thrust bearings and the balancing thrust bearing and counteracts the gas forces on the rotor of the machine in turn to reduce the forces transmitted through the main thrust bearings. The main thrust bearings are preloaded by a belleville washer which is disposed at an axial end of the rotor.
Referring to FIG. 1, a prior art bearing arrangement 10 included a pair of bearings 12, 14 which was bought as a set and which obtained a predetermined preload when clamped together. A thrust plate 16 and an inner ring 18 were ground together to assure even thickness and were interposed between the bearings 12, 14. The faces of the thrust plate 16 were not hardened. Initially, the bearings had no end shields and were exposed to corrosive compressed refrigeration or natural gas. This bearing arrangement proved to be troublesome, probably due to wear of the faces of the thrust plate 16 in combination with large variations of preload from the bearing manufacturers and exposure to the compressed gas. Subsequently, end shields 20 and helical compression springs 22 were added and the faces of the thrust plate were hardened. However, expected improvements in reliability were not realized. The springs 22 did develop a constant preload but if the position preload disappeared, and a small clearance developed between the bearing faces and the thrust plate 16, the rotor could fully axially move within the clearance space, thus causing vibration and fretting of the thrust bearing faces.
Referring to FIG. 2, a low speed (flooded) rotary screw machine has been sold by the assignee of the present application having a bearing arrangement 30 including a main thrust bearing 32 and a reverse thrust bearing 34 mounted in a face-to-face relationship on a rotor shaft 35. A first thrust plate 36 and a spacer 37 bear against the main thrust bearing 32 while a second thrust plate 38 bears against the reverse thrust bearing 34. The bearing arrangement 30 is captured between a shoulder 40 on the shaft 35 and a bearing nut 41. A preload condition is established on the bearings 32, 34 and is maintained by a set of helical springs 42 disposed in circumferentially-spaced recesses 43 in the first thrust plate 36 and by a wave spring 44 disposed in an annular recess 46 in the second thrust plate 38.